He was VERY surprised that myself, some English girls, the Irish girl, a couple of Americans from several states and a guy from Australia could all understand one another no problem.
It's not all that surprising at all, if you think about it. Due to the history of it's spread and it's use as the modern lingua franca, English in all it's forms is much more homogeneous than most others. In the strict linguistic sense, it's hard to point to a real, major, differentiated dialect.
Layer on modern Hollywood and American cultural influence, it should be no surprise most Americans are easily understood by most English speakers.
Now understand Scots, on the other hand, can be a chore. Especially the Glaswegians. I nominate Glascow as the home of the most incomprehensible English pronunciation ever.
And yeah, Austrian German can be weird. The first German I learned was a Bavarian German, so it wasn't so bad, but street signs and ordering food can get confusing even if Gruss Gott is a natural greeting for you.